Gas turbine efficiency is affected by the clearance between the tip of a turbine blade and the turbine casing. Clearance needs to be minimised for maximum turbine efficiency.
Turbine design calculations take into account all the related thermal expansions. The clearance is consequently set to avoid causing the tips of the blades to rub against the casing during certain manoeuvres. The design considerations ensure that the clearance is optimum at, for example, steady state operation. However, there is no control of the clearance during none steady state performance.